Converter devices, such as frequency converters, inverters and other devices modifying electrical power using semiconductor switches, employ semiconductors that switch currents and voltages. Typical examples of a semiconductor switches used in converter applications include an insulated gate bipolar transistor (IGBT) and a diode. IGBTs are able to switch high currents and high voltages. In many applications active switches, such as IGBTs, require an antiparallel connected diode that has to withstand similar currents and voltages as the switch itself. During each switching instant power loss occurs in the switch and this dissipated power heats up the switching component. Similarly, when current is cut-off from a diode, the diode heats up due to the dissipated power. The losses also occur during the conduction of the components.
In a voltage source frequency converter, which is a device used for controlling the operation of a motor, the outputted voltage is generated using an inverter. The inverter operates by forming short voltage pulses from a DC voltage such that the output voltage from the inverter is a pulsed voltage. The length of the pulses depends on the switching frequency of the inverter. The switching frequency has an impact on the control of the motor; the higher the switching frequency is, the better control dynamics are obtained.
Since each switching of the semiconductor component dissipates power, the higher switching frequency results in more losses and the cooling arrangement of the device should be designed carefully to meet the amount of losses such that the temperature of the semiconductor component does not exceed its highest allowable temperature.
In certain applications the converter device is loaded in a cyclic manner. In such use the converter is loaded heavily for a certain period of time and after the high load the load is reduced greatly. When this change of loading is continued, the semiconductor components are stressed heavily due to the variation of temperature in the semiconductor component. In a semiconductor component the actual pn-junction of the component heats the most as the power is dissipated in chip of the semiconductor. The cycling of temperature stresses the components heavily as different parts of the physical component heat differently and therefore the component is subjected to mechanical wear and premature breakdown.
It is known to limit the switching frequency of the converter device in order to limit the temperature changes in cyclic use. As mentioned above, the reduction of switching frequency of the converter reduces the losses in the component. Therefore, in a cyclic operation, the temperature variations may be reduced by reducing the switching frequency.
FIG. 1 shows an example of a known procedure for limiting the temperature variations in which switching frequency is limited based on the temperature of semiconductor component. FIG. 1 shows the limit of the switching frequency as a function of temperature. When the temperature of the semiconductor component exceeds a first fixed limit T_sf_low, the switching frequency is reduced linearly from its maximum value SF_max. Once the temperature increases further, the switching frequency is limited until the temperature reaches the second fixed limit T_sf_high after which the switching frequency is limited to value of SF_min. Thus depending on the determined temperature of the semiconductor component, the switching frequency is selected in the above manner.
FIG. 2 shows two different scenarios in connection with the known procedure. In the examples of FIG. 2 it is assumed that the converter has been idle for a long period and a stepwise load is given to the converter. In the first case the temperature of the cooling medium is 70° C. and thus the temperature of the semiconductor is also the same. The temperature of the semiconductor starts to increase rapidly until limit T_sf_low (100° C.) is reached. After the limit the switching frequency is decreased and the temperature rises in the end to 110° C. and the temperature variation dTj is 40° C. In the second case the temperature of the cooling medium is only 20° C. Although the temperature of the semiconductor rises as in the first case, the temperature does not exceed the lower limit T_sf_low. Therefore the whole loading period is operated without limiting the switching frequency. This leads to a situation in which the temperature variation dTj is 60° C.
The temperature limits T_sf_high and T_sf_low are set in such way that they operate with highest allowable temperature of the cooling medium. Further, the difference between the temperatures has to be chosen to be quite high so that the lower limit can be reached with other temperatures of the cooling medium than the highest allowable temperature. This, however, is not that effective as the smaller the difference between the temperature limits is, the more effectively the reduction of switching frequency compensates the temperature of the semiconductor component.
As shown above, the known system leads to a situation in which large temperature variations are possible as the main concern has been in the limiting of the maximum temperature. However, the larger variations in temperature wear the component more than smaller variations in higher absolute temperature.